GOG 



FOEMATION OF COEAL EEEFS. 



[Ch. XLIX. 



valves, just as tliej are often seen in barrier reefs. This 

 formation in the islands of Sandlewood, Sumbawa, Madura, 

 and Java, where it is exposed in sea cliffs, was found to be 



~ ~ "► or 300 feet thick, and it is believed to ascend 

 greater heights in the interior. It has usuallj the 



from 



much 



weathered surface turning nearly black.'^ 



It appears, therefore, premature to assert that there are no 

 recent coral formations uplifted to great heights, for we are 

 only beginning to be acquainted with the geological struc- 

 ture of the rocks of equatorial regions. Some of the upraised 

 islands, such as Elizabeth and Queen Charlotte, in the 

 Pacific, although placed in regions of atolls, are described by 

 Captain Beechey and others as flat-topped, and exhibitino- 

 no traces of lagoons. In explanation of the fact, we may 

 presume that, after they had been sinking for ages, the 

 descending movement was relaxed ; and while it was in the 

 course of being converted into an ascending one, the ground 

 remained for a long season almost stationary, in which case 

 the corals within the lagoon would build up to the surface 

 and reach the level already attained by those on the margin 

 of the reef. In this 



manner 



summ 



exam 



have not been noticed of fringing reefs uplifted above the 

 level of the sea. Mr. Darwin, indeed, cites one instance 



Mauritius 



moat 



of rare occurrence, for in the case of atolls or of barrier or 



must 



stroyed by denudation as soon as a reef begins to rise ; since 

 it is immediately exposed to the action of the breakers, and 

 the large and conspicuous corals on the outer rim of the atoll 

 or barrier are the first to be destroyed and to fall upon the 



bottom 



After slow and 



continued upheaval a wreck alone can remain of the original 

 reef. If, therefore, says Mr. Darwin, ' at some period as far 



* Paper read to Brit. Assoc. South- that these coral cliffs are now known to 

 ampton, 1846. Dr. Duncan informs me Mong to the Tertiary Period. 



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